In white light interferometers, the optical path difference between measuring beams and reference beams is varied in increments with the aid of a piezoelectric actuator. Detector images (camera images) are recorded in this process. The optical path difference in which interferences occur in the detector images is an indicator of the measured height value or distance value.
With most white light interferometers, the detector and the adjusting arrangement are operated side-by-side without synchronization and without feedback. Very cost-intensive white light interferometers regulate the adjusting arrangement with the aid of an external laser interferometer and thus keep the scanning of the measured object constant at each step, but with a free-running detector. One possibility for synchronizing the detector and the adjusting arrangement is to measure the displacement distance traveled and to trigger the detector as a function of this displacement distance at suitable scan points. However, this presupposes the use of a triggerable detector, but then only half the speed of the detector may be utilized because with the linear adjusters typically used as an adjusting arrangement, the fluctuation in the scan width is between 50% and 200%.
If the coupling between the scanning by the detector and the change in the optical path difference is imprecise, this affects the accuracy of the measurement results. Systematic vibrations in particular distort the result disproportionately. Tribological effects also have a particularly strong influence due to the low rate of change in the path difference, typically amounting to approximately 80 nm per camera cycle.